GRAPHENE TRANSISTOR:-
A graphene transistor is a nanoscale device based on graphene, a component of graphite with electronic properties far superior to those of silicon. The device is a single-electron transistor, which means that a single electron passes through it at any one time. A research team led by Professor Andre Geim of the Manchester Centre for Mesoscience and Nanotechnology built a graphene transistor and described it in the March 2007 issue of Nature magazine. Scientists have predicted that graphene transistors could scale to transistor channels as small as two nanometers (nm) with terahertz speeds. The base of the graphene transistor is graphene
Now before going to discuss about grapheme transistor(carbon nanotubes) lets take a brief introduction about GRAPHENE.
GRAPHENE
Introduction:-
Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. It can be viewed as an atomic-scale chicken wire made of carbon atoms and their bonds. The name comes from GRAPHITE + -ENE; graphite itself consists of many graphene sheets stacked together. Carbon is one of the most versatile chemical elements. Because it can form single, double and triple bonds, it forms thousands of chemical compounds, and has numerous elemental structures, or allotropes. The most common allotropes of carbon are diamond and graphite. Diamond consists of carbon atoms single-bonded to four other carbon atoms producing a tetrahedral crystal lattice. Its structure leads to its extreme hardness and thermal conductivity, but diamond is a very poor electrical conductor. In contrast, graphite consists of stacked layers of carbon sheets. Within an individual carbon sheet, known as graphene, the carbon atoms are sp2 hybridized and form a planar hexagonal lattice. The sp2 hybridization means that the carbons are ?-bonded in the plane, but are also ?-bonded above and below the plane. Graphene thus possesses one of the strongest bonds in nature and has a very high tensile strength. Graphene?s perpendicular p-orbitals lead to electron delocalization because there is no distinction between neighboring ? bonds, as indicated in Figure below.
Fig. Aromatic hydrocarbons like benzene shown here, share electrons in the p-orbitals with many neighboring atoms.
This conjugated ? orbital system permits the electrons to travel freely above and below the plane of carbon atoms with minimal scattering. Because of the minimal scattering and strong delocalization of the electrons, graphite is a good conductor along the plane. However, in graphite, electrostatic forces bind the layers together only very weakly, and graphite is a very soft mineral. In addition, the other layers interfere with the behavior of the single sheets, even if not strongly. An ideal system would be to study free single-layer graphene, but until a few years ago, two-dimensional systems like free graphene were believed to be impossible.
In recent years, the two most familiar allotropes of carbon have been joined by a number of newly discovered graphene-like materials. The first major graphene-related substance discovered was C60, also known as buckminsterfullerene, buckyball, and fullerene, a soccer-ball-like configuration of carbon atoms found in common lamp soot and known to be very stable. Soon, the scientific community encountered similar fullerene-type carbon structures called a carbon nanotubes. Carbon nanotubes are needle-like tubes of rolled up graphene sheets that exhibit many unusual and useful properties such as extreme tensile strength and high conductivity. .
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A researcher at Stanford University has provided strong experimental evidence that ribbons of carbon atoms can be used for future generations of ultrafast processors.
Hongjie Dai, a professor of chemistry at Stanford, and his colleagues have demonstrated a new chemical process that produces extremely thin ribbons of a carbon-based material called graphene. He has demonstrated that these ribbons, once incorporated into transistors, show excellent electronic properties. Such properties have been predicted theoretically, Dai says, but not demonstrated in practice. These properties make graphene ribbons attractive for use in logic transistors in processors.
The discovery could lead to even greater interest in the experimental material, which has already attracted the attention of researchers at IBM, HP, and Intel. Graphene, which consists of carbon atoms arranged in a one-atom-thick sheet, is a component of graphite. Its structure is related to carbon nanotubes, another carbon-based material that's being studied for use in future generations of electronics. Both graphene and carbon nanotubes can transport electrons extremely quickly, which could allow very fast switching speeds in electronics. Graphene-based transistors, for example, could run at speeds a hundred to a thousand times faster than today's silicon transistors.
But graphene sheets have one significant disadvantage compared with the silicon used in today's chips. Although graphene can be switched between different states of electrical conductivity--the basic characteristic of semiconductor transistors--the difference between these states, called the on/off ratio, isn't very high. That means that unlike silicon, which can be switched off, graphene continues to conduct a lot of electrons even in its "off" state. A chip made of billions of such transistors would waste an enormous amount of energy and therefore be impractical.
Researchers had theorized, however, that it might be possible to dramatically improve these on/off ratios by carving graphene sheets into very narrow ribbons just a few nanometers wide. There had been early evidence supporting these theories from researchers at IBM andColumbia University, but the ratios produced were still much lower than those in silicon.
Dai decided to take a different approach to making thin graphene ribbons. Whereas others had used lithographic techniques to carve away carbon atoms, Dai turned to a solution-based approach. He starts with graphite flakes, which are made of stacked sheets of graphene. Then he chemically inserts sulfuric acid and nitric acid molecules between these flakes and rapidly heats them up, vaporizing the acids and forcing the graphene sheets apart. "It's like an explosion," Dai says. "The sheets go separate ways, and the graphite expands by 200 times."
Next, he suspends the now-separated sheets of graphene in a solution and exposes them to ultrasonic waves. These waves break the sheets into smaller pieces. Surprisingly, Dai says, the sheets fracture not into tiny flakes but into thin and very long ribbons. These ribbons vary in size and shape, but their edges are smooth--which is key to having consistent electronic properties. The thinnest of the ribbons are less than 10 nanometers wide and several micrometers long. "I had no idea that these things could be made with such dimensions and smoothness," Dai says.
When Dai made transistors out of these ribbons, he measured on/off ratios of more than 100,000 to 1, which is attractive for transistors in processors. Previously, room-temperature on/off ratios of graphene ribbons had been measured at about 30 to 1.
Still, many obstacles remain to making graphene processors using Dai's methods, says Walter de Heer, a physics professor at Georgia Tech. The ribbons made with Dai's process have to be sorted. Pieces that are too large or not in the shape of ribbons have to be weeded out. There also needs to be a way of arranging the ribbons into complex circuits.
However, researchers already have ideas about how to address these challenges. For example, graphene ribbons have more exposed bonds at their edges, so chemicals could be attached to these bonds that would direct the ribbons to bind to specific places to form complex circuits, de Heer says.
The best way to make graphene electronics, however, may be to take advantage of the fact that graphene can be grown in large sheets, says Peter Eklund, a professor of physics at Penn State. If better lithography methods are developed to pattern these sheets into narrow ribbons and circuits, this could provide a reliable way of making complex graphene-based electronics.
Ultimately, the most important aspect of Dai's work could be the fact that it has demonstrated electronic properties that were only theoretical before, Eklund says. And this could lead to even more interest in developing graphene for next-generation computers. "Once you get a whiff of narrow graphene ribbons with a high on/off ratio, this will tempt a lot of people to try to get in there and either make ribbons by high-technology lithographic processes, or try to improve the approach developed by Dai," says Eklund.
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Graphene transistors promise 100GHz speeds
By Adam Stevenson | Last updated about a year ago
Researchers are running into the physical limits of speed and scaling in silicon transistor technology, forcing them to look elsewhere for next-generation devices. The leading candidate to replace silicon being pursued by, well, pretty much everyone, is graphene. Graphene, single sheets of graphitic carbon, is exciting because it is a single atom thick and has remarkably high electron mobilities (100 times greater than silicon), making it ideally suited to atomic-scale, high-speed operation. Also, graphene's electrical properties can be controlled, switching it among conducting, semiconducting and electrically insulating forms. That means graphene-only (or, more likely, graphene-mostly) devices are, in principle, possible.
In this week's Science, researchers from IBM demonstrate graphene-based field effect transistors (FETs) that may operate at much higher speeds (100GHz) than Si FETs. Graphene layers were thermally grown on two-inch SiC wafers and the FETs were formed using standard Si fabrication techniques with HfO2 as the gate oxide. That's a rather significant point?the researchers actually created an entire wafer of these devices.
The smallest gate length demontrated in the paper was 240nm, quite large compared to current generation Si (32nm), but the graphene was one or two layers (meaning one or two atoms) thick in all the tested devices?a considerable improvement over Si.
High frequency operation, colloquially referred to as the speed of the transistors, was the key property examined in the paper. As operating frequency increases, electrons have less time to respond to the electrical fields that drive transistors, which will eventually cause the transistor to fail because the electrons simply can't conduct across the material fast enough.
The graphene FETs in this work were tested up to 30GHz and, extrapolating those results, the authors showed that the FETs would operate, albeit poorly, up to 100GHz. Similarly sized Si devices are limited to 30GHz operation. Assuming these devices can be scaled, they will undoubtedly present a dramatic speed increase over current generation Si.
Because the graphene used in this study was conductive (i.e. no band gap), the demonstrated voltage-current characteristics were strange compared to Si. Specifically, current continued to increase linearly with drain voltage up to device breakdown. Si-based transistors typically have a point, called the threshold, at which a current cannot increase despite increasing drain voltage.
This study is a mixed bag of promise and hype. The 100GHz speed touted in the article's title is an extrapolation?no such properties were actually measured. Also, the electron mobilities, the key property for high frequency operation, that the authors measured in the fabricated devices were pedestrian compared to graphene's potential, probably due to the thermal process used to synthesized the graphene layer. Future devices could dramatically outperform these FETs if wafer-scale fabrication can replicate some of the better electron mobility measurements of graphene.
Graphene devices have grown by leaps and bounds over the past few years, and they are probably the best bet to eventually replace silicon. Demonstrations like this are important because they show that wafer-scale production is possible, and the properties, while not ideal, are truly impressive, in that they're already beginning to push the limits of Si technology.
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